Oxidation-Reduction (Redox) Reactions
Balance complete redox half-reactions in the stated acidic or basic medium and cancel electrons.
AP Chemistry Topic 4.9 · CED printed p. 87 / PDF p. 94. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Moles and molarity
- Ionic charges and molecular formulas
- Intermolecular versus intramolecular interactions
- Ideal gas equation with kelvin temperature
A half-reaction keeps electron transfer visible while you balance atoms and charge. First balance elements other than H and O. In acidic solution, use H2O to balance O and H+ to balance H, then electrons to balance charge. Multiply half-reactions until lost and gained electrons match; add and cancel identical terms. In basic solution, you can first balance as acidic, then add equal OH− to both sides to remove H+ as water and cancel extra water. This is accounting for the specified solution medium, not permission to invent a reactant. Electrons must cancel from the final overall equation. Finish with an atom inventory and charge check; a visually impressive equation can still be wrong. For simple metal/ion reactions, inspection may be enough, but showing the two half-reactions explains where the electrons go.
Let's work through it
Balance permanganate oxidation of Fe2+ to Fe3+ in acidic solution, with MnO4− reduced to Mn2+.
- Reduction: MnO4− + 8H+ + 5e− → Mn2+ + 4H2O. Oxygen gives four waters; eight protons balance H; five electrons make each side +2.
- Oxidation: Fe2+ → Fe3+ + e−. Multiply by five to supply five electrons.
- Add: MnO4− + 8H+ + 5Fe2+ → Mn2+ + 4H2O + 5Fe3+. Charge is +17 on both sides; all atoms match.
Now let’s balance the oxidation half of Cl₂ →ClO⁻ in basic solution instead of skipping to the finished result.
- Start by balancing two chlorine atoms: Cl₂ →2ClO⁻. Add 2H₂O on the left to supply two O atoms and 4H⁺ on the right to balance four H atoms.
- The right side currently has charge −2+4 =+2. Add 2e⁻ on the right to bring its charge to zero: Cl₂ +2H₂O →2ClO⁻ +4H⁺ +2e⁻. This is the acidic bookkeeping form.
- Basic medium requires eliminating H⁺. Add 4OH⁻ to both sides, preserving equality: Cl₂ +2H₂O +4OH⁻ →2ClO⁻ +4H₂O +2e⁻.
- Cancel 2H₂O from both sides: Cl₂ +4OH⁻ →2ClO⁻ +2H₂O +2e⁻. Left charge is −4 and right charge is −2−2 =−4.
- Add the reduction half Cl₂ +2e⁻ →2Cl⁻. Cancel electrons and divide all coefficients by two: Cl₂ +2OH⁻ →Cl⁻ +ClO⁻ +H₂O. The final equation has no H⁺ and charge −2 on each side.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
Why multiply one half-reaction before adding?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
Why multiply one half-reaction before adding?
Worked answer and reason
Electrons lost must exactly equal electrons gained. Multiply every term, not only the electrons.
The coupled redox event conserves charge and matter.
Retrieval 2
Can electrons remain in the final chemical equation?
Worked answer and reason
No, for an overall redox reaction they cancel because one half transfers exactly what the other receives.
Half-reactions are components of the same coupled transfer.
Catch the tempting mistake
- Balance atoms and ignore charge.
- Add electrons in half-reactions and verify total charge after cancellation.
- For basic solution leave H⁺ in the final equation.
- Convert the balanced acidic form with equal OH⁻ on both sides, then cancel water.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 4 problem 17 · ap application · symbolic
- Unit 4 problem 18 · ap synthesis · symbolic
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For the teacher
- Require students to write the charge totals next to their final equation.
- Use a basic-medium conversion to check that the algorithm has meaning.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 4.9.A
- Represent a balanced redox reaction equation using half-reactions.
- 4.9.A.1
- Balanced chemical equations for redox reactions can be constructed from half-reactions.
Science references for this teaching
- College Board AP Chemistry Course and Exam Description, Fall 2024 — Exact unit learning requirements and exclusions; supplied PDF is pinned by the integration catalog.
- OpenStax Chemistry 2e: Writing and Balancing Chemical Equations — Scientific check: conserved atoms/charge and complete/net ionic conventions.
- OpenStax Chemistry 2e: Classifying Chemical Reactions — Scientific check: precipitation, oxidation-number conventions, proton transfer; AP exclusions applied separately.
Glossary and deeper reading · 6 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Mole
A mole is an amount of substance containing exactly 6.02214076 × 10²³ specified entities. The entities can be atoms, molecules, ions, or formula units, so the counted entity must be named. For example, one mole of oxygen molecules contains two moles of oxygen atoms.
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Molarity
Molarity is the amount of a solute in moles divided by the final solution volume in liters. It describes concentration rather than the total amount of solute. For example, 0.10 mole dissolved to make 0.50 liter of solution gives a molarity of 0.20 mole per liter.
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Molecular formula
A molecular formula gives the actual number of each kind of atom in a molecule. It can be a whole-number multiple of the empirical formula. For example, glucose has molecular formula C₆H₁₂O₆ and empirical formula CH₂O.
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Ideal gas law
The ideal gas law relates pressure, volume, amount, and absolute temperature in an idealized gas model. It treats particles as having negligible volume and no intermolecular attractions. For example, at fixed amount and temperature, doubling the container volume halves the ideal gas pressure.
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Half-reaction
A half-reaction describes either the oxidation or reduction part of a redox process. It includes electrons explicitly so that charge and electron transfer can be balanced. For example, zinc forming a zinc ion and two electrons is an oxidation half-reaction.
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Ion
An ion is an atom or group of atoms with a net electric charge. Ordinary ion formation changes electron count without changing the element's proton count. For example, a sodium atom losing one electron becomes a sodium ion with charge +1.